Clutches and similar types of load-transmitting mechanisms are highly susceptible to wear during the transition period when the clutch is engaging and disengaging, in which the input and output members are being coupled or uncoupled. While friction clutches are generally provided with a sacrificial wear surface which can be replaced as required, positive-type clutches are designed to transmit torque without allowing for any significant degree of slippage between the engaging members. A variation of the positive-type clutch is an overrunning clutch, which positively engages the input and output members, yet allows the output member to rotate relative to the input member when the output member is urged to rotate faster than the input member. Due to its mode of operation, an overrunning clutch must be capable of repeatedly withstanding loading with minimal wear and damage to its load bearing surfaces.
An example of an application for an overrunning clutch is in turbochargers employed with two-stroke diesel engines used for locomotives. At lower engine speeds and loads, the turbocharger must be mechanically driven in that the exhaust gases do not contain enough energy to drive the turbocharger. However, sufficient energy is present in the exhaust gases at higher engine speeds and loads for driving the turbocharger, necessitating the use of an overrunning clutch to disengage the turbocharger from the gear train in order to allow the turbocharger to respond to the input provided by the exhaust gases. Clutches designed for this application generally include an annular-shaped camplate at whose outer periphery are formed a number of slots. The base of each slot defines a ramp which is in rolling contact with a cylindrical roller, such that the ramps serve as a load bearing surfaces for the camplate. The clutch must be capable of withstanding high bearing loads while continuously and reliably operating for extremely long periods of time, in that locomotive diesel engines are often in operation in excess of 90 percent of their lifetime.
As one would expect, the wear characteristics of the rollers and the ramps on the camplate largely determine the service life of the clutch. The rollers are preferably formed from a hard material, such as SAE 4620 or 8620H, and are carburized to a depth sufficient to provide a durable wear surface. The camplate is also preferably formed from a hard material, such as SAE 8620H, and carburized to a depth which is sufficient to achieve a surface hardness which is compatible with that of the rollers. Even so, the ramps of the camplate tend to deform slightly under the load of their rollers during operation. Significantly, the edges of the ramps inherently suffer higher contact stresses due to greater deformation at the ends of the rollers under normal loading conditions. Contact stresses are even higher when misalignment exists between the rollers and the ramps, such as when the rolling axis of a roller is not parallel with the axis of the camplate. Consequently, greater wear typically occurs at the edges of the camplate ramps.
It is generally known to deposit functional chromium, or hard chrome, plating on the load bearing surfaces of gears, bearings, and camming mechanisms in order to produce very hard, gall and wear resistant surfaces. However, electroplating processes naturally tend to produce metal buildup along edges due to concentration of the electrostatic flux. Unless removed, this buildup promotes cracking and chipping of the chromium plating, which leads to premature failure of the plated surface as well as the member bearing against the plated surface. To eliminate the plating buildup, a post-plating grinding operation is necessary. However, grinding the chromium plating carries a significant cost penalty in terms of processing time and tooling. As a result, profile grinding of a plated surface is not typically done. Though it is known to profile grind carburized surfaces, such an approach has typically been adopted only where there is a significant concern for contact stresses, such as with gears and bearings.
When the mating member is a bearing, an alternative to profiling the plated load bearing surface is to form the bearing as a cylindrical roller having a larger diameter midsection and smaller diameter ends. With this approach, the distribution of contact stresses over the plated surface is more uniform than if a standard roller bearing is used. However, the cost to manufacture cylindrical rollers having sufficient taper to adequately distribute the contact stresses on the ramp surface of a camplate is also quite high.
An alternative to the approaches mentioned above is to alter the profile of the load bearing surface by profile grinding the edges of the bearing surface prior to plating. A layer of chromium is then deposited on the load bearing surface so as to generally generate a crowned profile on the surface, which promotes a more uniform distribution of contact stresses across the load bearing surfaces. Again, however, the significant costs associated with the grinding operation are a disadvantage to adopting this approach.
While the above profile grinding operations have generally been utilized by gear and bearing manufacturers, the processing and cost disadvantages associated with electroplating and profile grinding have tended to discourage those in the railway industry from chromium plating the ramps of clutch camplates for turbochargers of locomotive engines. Though improvements in camplate service life are obviously desirable, economic efficiencies dictate that a method employed to improve clutch life must be evaluated with consideration for the costs involved in implementing the method. Generally, in applications where chromium plating of the camplate ramps is warranted, the additional expense of a profile grinding operation may not be cost effective.
From the above, it can be seen that what is needed is a cost-effective method by which the service life of a clutch could be improved, wherein the method is particularly adapted to improving the wear characteristics of the load bearing surfaces of a clutch used to engage and disengage the turbocharger of a locomotive diesel engine.